In view of increased demand for four-wheel drive vehicles, many different power transfer systems are currently being incorporated into vehicular driveline applications for transferring drive torque to the wheels. In some vehicles, a power transmission device is operably installed between the primary and secondary drivelines. Such power transmission devices are typically equipped with a torque transfer mechanism for selectively and/or automatically transferring drive torque from the primary driveline to the secondary driveline to establish a four-wheel drive mode of operation. For example, the torque transfer mechanism may include a dog-type lock-up clutch that can be selectively engaged for rigidly coupling the secondary driveline to the primary driveline to establish a locked or “part-time” four-wheel drive mode. In contrast, drive torque is only delivered to the primary driveline when the lock-up clutch is released for establishing a two-wheel drive mode.
A modern trend in four-wheel drive motor vehicles is to equip the power transmission device with a transfer clutch in place of the lock-up clutch. The transfer clutch is operable for automatically directing drive torque to the secondary wheels, without any input or action on the part of the vehicle operator, when traction is lost at the primary wheels for establishing an “on-demand” four-wheel drive mode. Typically, the transfer clutch includes a multi-plate clutch assembly that is installed between the primary and secondary drivelines and a clutch actuator for generating a clutch engagement force that is applied to the multi-plate clutch assembly. In passive-type transfer clutch applications, the clutch actuator generates the clutch engagement force in response to the magnitude of the speed difference between the primary and secondary wheels. In active-type applications, however, the clutch actuator includes a power-operated device that is actuated in response to electric control signals sent from an electronic control unit (ECU). The ECU receives input signals from speed sensors associated with the primary and secondary drivelines as well as from other vehicle sensors and generates the control signal based thereon. Thus, such “on-demand” power transmission devices can automatically respond to slip conditions which occur during different types of driving situations and road conditions.
A large number of on-demand power transmission devices have been developed with an electrically-controlled clutch actuator that can regulate the amount of drive torque transferred to the secondary driveline as a function of the value of the electrical control signal applied thereto. In some applications, the transfer clutch employs an electromagnetic clutch as the power-operated clutch actuator. For example, U.S. Pat. No. 5,407,024 discloses an electromagnetic coil that is incrementally activated to control movement of a ball-ramp drive assembly for applying a clutch engagement force on the multi-plate clutch assembly. Likewise, Japanese Laid-open Patent Application No. 62-18117 discloses a transfer clutch equipped with an electromagnetic actuator for directly controlling actuation of the multi-plate clutch pack assembly.
As an alternative, the transfer clutch can employ an electric motor and a drive assembly as its power-operated clutch actuator. For example, U.S. Pat. No. 5,323,871 discloses an on-demand transfer case having a transfer clutch equipped with an electric motor that controls rotation of a sector plate which, in turn, controls pivotal movement of a lever arm that is operable for applying the clutch engagement force to the multi-plate clutch assembly. Moreover, Japanese Laid-open Patent Application No. 63-66927 discloses a transfer clutch which uses an electric motor to rotate one cam plate of a ball-ramp operator for engaging the multi-plate clutch assembly. Finally, U.S. Pat. Nos. 4,895,236 and 5,423,235 respectively disclose a transfer case equipped with a transfer clutch having an electric motor driving a reduction gearset for controlling movement of a ball screw operator and a ball-ramp operator which, in turn, apply the clutch engagement force to the clutch pack.
As noted, conventional on-demand power transmission devices typically operate in the two-wheel drive mode and are adaptively shifted into the four-wheel drive mode in response to lost traction at the primary wheels. In contrast, “full-time” power transmission devices utilize a center or interaxle differential between the primary and secondary drivelines to continuously transfer drive torque therebetween while also accommodating speed differentiation between the drivelines. To minimize loss of traction due to wheel slippage, many full-time power transmission devices are also equipped with a biasing clutch for limiting interaxle slip and varying the distribution ratio of the drive torque transmitted across the interaxle differential to the primary and secondary drivelines. Like the on-demand transfer clutch, many biasing clutches include a multi-plate clutch assembly and a power-operated clutch actuator that is adaptively controlled by a control system to vary engagement of the clutch assembly.
While many power-operated clutch actuation systems similar to those described above are currently used in on-demand and full-time four-wheel drive vehicles, a need exists to advance the technology and address recognized system limitations. In an effort to address such concerns, new technologies are being considered for use in vehicle control applications.